Protective coatings are essential to the satisfactory performance of gas turbine engines. In particular, in the turbine section of an engine various components must withstand high stress while enduring a corrosive gas stream whose temperatures may be as great as 2500.degree. F. As demands for efficiency and performance increase, the requirements for coating durability increase.
The most effective coatings for protecting superalloy turbine components are those known as MCrAlY coatings where M is selected from the group consisting of iron, nickel, cobalt and certain mixtures thereof. Such coatings are also referred to as overlay coatings because they are put down in a predetermined composition and do not interact significantly with the substrate during the deposition process. U.S. Pat. No. 3,528,861 describes a FeCrAlY coating as does U.S. Pat. No. 3,542,530. U.S. Pat. No. 3,649,225 describes a composite coating in which a layer of chromium is applied to a substrate prior to the deposition of a MCrAlY coating. U.S. Pat. No. 3,676,085 describes a CoCrAlY overlay coating while U.S. Pat. No. 3,754,903 describes a NiCrAlY overlay coating. U.S. Pat. No. 3,928,026 describes a NiCoCrAlY overlay coating having particularly high ductility.
A variety of alloying additions have been proposed for use with the MCrAlY compositions. U.S. Pat. No. 3,918,139 describes the addition of from 3 to 12% of a noble metal. U.S. Pat. No. 4,034,142 describes the addition of from 0.5 to 7% silicon to a MCrAlY coating composition. Finally, U.S. Pat. No. 3,993,454 describes an overlay coating of the MCrAlHf type.
U.S. Pat. No. 4,078,922 describes a cobalt base structural alloy which derives improved oxidation resistance by virtue of the presence of a combination of hafnium and yttrium.